Uniform fatigue life spherical elastomeric bearing

ABSTRACT

An elastomeric spherical bearing includes a multiple of elastomeric layers with an essentially equivalent fatigue life.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under N00019-06-C-0081 awarded by The United States Navy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to an elastomeric bearing.

One goal of elastomeric spherical bearing design is the smallest/lightest package that meets the desired design life requirements. The design life is typically determined through a single motion strain. One conventional elastomeric bearing design methodology discloses how to develop a bearing with a uniform steady compression induced strain, γTc, and a uniform strain distribution for one motion pitch γ_(θ) or flap γ_(β). Such conventional elastomeric bearing design methodology, however, does not account for coupled load and motion or for the fatigue damage from additional motion strains. That is, the actual life of each layer is not dependent exclusively on pitch or flap strain as pressured by this conventional methodology. Furthermore, the conventional methodology generates a bearing that does not provide uniform life at each layer and may therefore result in a relatively inefficient elastomeric bearing.

SUMMARY OF THE INVENTION

An elastomeric spherical bearing according to an exemplary aspect of the present invention includes a multiple of elastomeric layers, each of said multiple of elastomeric layers having an essentially equivalent fatigue life.

A method of calculating a uniform life spherical elastomeric bearing according to an exemplary aspect of the present invention includes adjusting each layer thickness to produce a uniform fatigue life of each bearing layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently disclosed embodiment. The drawings that accompany the detailed description can be briefly described as follows:

FIG. 1 is a perspective view a rotor head assembly utilizing an elastomeric bearing according to one non-limiting embodiment of the present invention;

FIG. 2A is an enlarged broken-away perspective view of the elastomeric bearing in combination with a rotor assembly yoke and shear segment of the rotor hub assembly of FIG. 1;

FIG. 2B depicts an enlarged view of the elastomeric laminates of the elastomeric bearing of FIG. 2A;

FIG. 3 is a schematic view of a section of one segment of an elastomeric layer of the elastomeric bearing;

FIG. 4 is graph representing the shear modulus and elastomer thickness of one elastomeric bearing with three layers according to one non-limiting embodiment of the present invention; and

FIG. 5 is a graph of the elastomeric bearing of FIG. 4 illustrating an essentially equivalent elastomeric layer life.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT

Referring to FIG. 1, a rotor hub assembly 10 typical of a rotary-wing aircraft includes a hub retention member 12 which drives a plurality of rotor blade assemblies 14 about an axis of rotation 16.

The hub retention member 12 includes a plurality of radial spokes 20 and shear segments 22. Each shear segment 22, in combination with its respective radial spokes 20, form a structural loop for accepting a rotor assembly yoke 24. The yoke 24 is generally C-shaped and circumscribes, in looped fashion, the respective shear segment 22. The yoke 24 is disposed in combination with a cuff structures 28 which, in turn, mount to the root end of each rotor blade assembly 14.

A spherical elastomeric bearing assembly 30 is interposed between each rotor assembly yoke 24 and the respective shear segment 22 to accommodate the multi-directional displacement of the rotor blade assembly 14.

Referring to FIG. 2A, the spherical elastomeric bearing 30 is shown in combination with a rotor assembly yoke 24 and a respective shear segment 22. The spherical elastomeric bearing 30 includes a central bearing element 32 having a spherical bearing surface 32 s which defines a bearing focal point 30 f. The bearing focal point 30 f defines the flap, lead-lag and pitch axes, Fa, La, and Pa, respectively, about which the rotor blade assembly articulates.

To the spherical surface 32 s is bonded discrete spherical elastomeric elements 34 about the bearing focal point 30 f. Furthermore, each spherical elastomeric element 34 includes a multiple of alternating layers (see FIG. 2B) of elastomer 36 and nonresilient shims 38, respectively, which are disposed at increasing radii from the bearing focal point 30 f and have a center of curvature C.sub.c which is coincident therewith.

Each elastomeric layer 36 of the elastomeric spherical bearing 30 provides a uniform fatigue life as will be further described below. It should be understood that although a particular rotor hub application is illustrated in the disclosed non-limiting embodiment, elastomeric bearing for any application including but not limited to aerospace, heavy machinery, and civil engineering (bridges, buildings, etc.) will benefit herefrom.

Each spherical elastomeric bearing layer 36 has a fatigue curve with alternating strain (S) as the ordinate, and cycles to failure (N) as the abscissa (S−N). The fatigue life of each elastomer bearing layer 36 can be approximated from the S−N curves by the following equations (1)-(5):

Pitch Angle Motion+/−θ

$\begin{matrix} {N_{\theta} = \left( \frac{C}{\gamma_{\theta}} \right)^{\alpha}} & (1) \end{matrix}$

where γ_(θ) equals the pitch strain as a percentage and N_(θ) equals the number of cycles to failure. C and α are constants (general values below) that are derived from curve fitting methods to approximate the particular S−N curve.

C˜5000;

α˜3.5 to 5.0

Flap Angle Motion+/−3

$\begin{matrix} {N_{\beta} = \left( \frac{0.8C}{\gamma\beta} \right)^{\alpha}} & (2) \end{matrix}$

Coupled T_(c) and Flap Shear T_(c)·sin β_(i)

$\begin{matrix} {{N\; \gamma \; s} = \left( \frac{0.576C}{\gamma_{s}} \right)^{1.6\alpha}} & (3) \end{matrix}$

Where γ_(s) is the shear strain

The life of a layer is calculated from the combined cumulative damage of the two motions and the shear load.

$\begin{matrix} {{NN} = {\frac{1}{N_{\theta}} + \frac{1}{N_{\beta}} + \frac{1}{N_{\gamma \; s}}}} & (4) \end{matrix}$

Such that fatigue life:

$\begin{matrix} \frac{1}{NN} & (5) \end{matrix}$

Referring to FIG. 3, a section of one layer 36A of the elastomeric spherical bearing 30 is schematically illustrated such that nomenclature may be defined. The section 36A includes an outwardly facing convex surface 40 and an inwardly facing concave surface 42. It should be understood that the section 36A is representative of a single layer of an elastomeric material which may be attached to another layer (not shown)

Fatigue Loading:

-   -   Pitch Angle Motion+/−θ;     -   Flap Angle Motion+/−β;     -   Shear Loading+/−T_(c)·sin β_(i)         -   Where T_(c) is the compression load.

Initial Practice Design Constraints:

-   -   Shear Modulus G_(1stLayer)=240 psi     -   Compression Shear Strain γ_(Tc)=100% For the first layer     -   Pitch Strain+/−β_(θ)=35% For All Layers

First Layer:

-   -   Compressive Shear Strain: γ_(Tc)=100%     -   Calculate Pitch Strain γ_(θ) and Flap Strain γ_(β)     -   Normalize Pitch Strain γ_(θ) to 35%     -   Calculate Normalized γ_(β) to β_(i)

γ_(vi) =f(T _(c) sin β_(i))

-   -   Calculate

Adjust layer thickness, t₁ to obtain desired life [iterate using approach 1, “Local Flow chart Methodology” (below)].

Repeat until Pitch Strain, γ_(θ)=35% For all layers

Local Flow Chart Methodology—Approach #1—Constant+/−γ_(θ) Pitch Strain Percentage

Design Bearing for a Uniform Vibratory Pitch Shear Strain by varying the layer shear modules by:

$\begin{matrix} {G_{i} = \frac{G_{i - 1}R_{i - 1}^{3}\varphi_{i - 1}\sin \; \beta_{i}}{R_{i}^{3}\varphi_{i}\sin \; \beta_{i - 1}}} & (6) \end{matrix}$

Where

G_(i) is the elastomer layer shear modulus;

G_(i-1) is the previous elastomer layer shear modulus (starting at the layer closest to the focal point);

R_(i) is the mean radius of the layer;

R_(i-1) is the mean radius of the previous layer (starting at the layer closest to the focal point); and

φ_(i)=cos A _(i)·sin A _(i) ²+2·cos A _(i)−cos B _(i)·sin B _(i) ²−2·cos B _(i)  (7)

Where A_(i) is the Inner Angle and B_(i) is the outer angle.

For a given fatigue Life

$\begin{matrix} {\frac{1}{N_{\beta}} = \frac{1}{\frac{1}{Life} - \frac{1}{N_{\theta}}}} & (8) \end{matrix}$

Using Equations (1)-(8)

$\begin{matrix} {{\hat{\gamma}}_{s} = {{\left\lbrack \left\{ {\frac{1}{N\; \hat{\beta}}\hat{\beta}} \right\}^{\frac{1}{\alpha}} \right\rbrack^{- 1}\left( {0.8C} \right)} - \gamma_{\beta}}} & (9) \\ {\gamma_{s} = \frac{0.576\; C}{\left\{ \frac{0.8C}{{\hat{\gamma}}_{s}} \right\}^{\frac{1}{1.6}}}} & (10) \end{matrix}$

Adjusting the layer thickness such that the actual bearing shear strain equals the above calculated shear strain constraint (equation 10), will produce a uniform fatigue life for each bearing layer.

Utilizing the Equations described above, one calculation procedure according to one non-limiting embodiment of the present invention is as follows:

-   -   1.) Determine Loads and Motions.         -   a. Determine the axial load. The axial load acts in a             direction parallel to the Z axis of rotation. For             helicopters, this is generally the centrifugal force from             the rotor blade.         -   b. Determine the radial load. The radial load acts in the R,             radial direction. This is usually the shear load on the             rotor blade.         -   c. Determine the vibratory pitch motion (theta) angle. This             motion rotates about the Z axis and is sometimes referred to             as the torsional rotation. This motion is a vibratory             motion. The angle is a plus or minus motion from the origin.         -   d. Determine the flapping (cocking) motion angle.     -   2.) Determine Fatigue Life Desired (In Number Of Cycles Or         Hours).     -   3.) Enter Bearing Dimensions.         -   a. Inner radius (Ri), Inner Angle (Bi), Inner Angle (Ai),             Side Angle (Alpha) sometimes referred to as outer taper             angle, and Side Angle (Blpha) sometimes referred to as inner             taper angle.         -   b. Enter Shim thickness     -   4.) Enter Bearing Elastomer Material Properties.         -   a. Enter Bulk modulus of elastomer, (Kb, psi). A value of             200,000 psi is good practice.         -   b. Enter the first layer shear modulus (G, psi). A value of             240 psi is good practice.     -   5.) Calculate Geometries. Inner And Outer Angles, Inner And         Outer Radius, Mean Radius, Etc.     -   6.) Calculate Shape Factors Per Layer.     -   7.) Calculate Effective Compression Modulus (Ec)     -   8.) Calculate Radial And Axial Strains To Get Compression Shear         Strain. Change Layer Thickness To Achieve Compression Strain         Allowable.     -   9.) Calculate Stiffnesses (Radial, Axial, Flapping, And         Torsional Stiffnesses)     -   10.) Calculate Torsional Strain. Add More Layers To Achieve         Allowable.     -   11.) The Shear Modulus Is Calculated From The Geometry To         Achieve A Uniform Torsional Strain. (See Equation #6 For Shear         Modulus)     -   12.) Calculate Flapping Strain     -   13.) Calculate Shear Strain Allowable γ_(s).     -   14.) Calculate Life     -   15.) Vary The Thicknesses Of Each Layer To Meet Shear Strain         Allowable and to Achieve Uniform Life.

It should be understood that the instructions are basically the same for a solid bearing and a bearing with a central opening. It should also be understood that an elastomeric bearing with any number of layers may be calculated by the method herein. By way of illustration, an elastomeric bearing 30A with three layers according to one non-limiting embodiment, has inputs delineated in the chart below:

To provide the following properties:

These properties provide a shear modulus and elastomer thickness by layer (FIG. 4) such that each layer the elastomeric bearing 30 has an essentially equivalent life (FIG. 5).

It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.

It should be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit from the instant invention.

Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention.

The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The disclosed embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention. 

1. An elastomeric spherical bearing comprising: a multiple of elastomeric layers; and a shim mounted between at least two of said multiple of elastomeric layers, each of said multiple of elastomeric layers having an essentially equivalent fatigue life.
 2. The elastomeric spherical bearing as recited in claim 1, wherein an inner layer of said multiple of elastomeric layers is mounted to a central bearing element having a spherical bearing surface.
 3. The elastomeric spherical bearing as recited in claim 2, wherein an inner layer of said multiple of elastomeric layers is mounted to a rotor assembly component.
 4. The elastomeric spherical bearing as recited in claim 2, wherein an outer layer of said multiple of elastomeric layers is mounted to a cuff structures of a rotor assembly.
 5. A method of calculating a uniform life spherical elastomeric bearing comprising: adjusting each layer thickness of an elastomeric bearing having uniform pitch strain to satisfy: $\gamma_{s} = \frac{0.576C}{\left\lbrack {\left( {0.8C} \right)\left\lbrack {{\left\lbrack \left\lbrack {\frac{1}{Life} - \left( \frac{C}{\gamma_{\theta}} \right)^{- \alpha}} \right\rbrack^{\frac{1}{\alpha}} \right\rbrack \left( {0.8C} \right)} - \gamma_{\beta}} \right\rbrack}^{- 1} \right\rbrack^{\frac{1}{1.6}}}$ to produce a uniform fatigue life of each bearing layer where: C and α are constants derived from curve fitting methods to approximate the particular S−N curve; L_(ife) is the desired fatigue life; γ_(θ) is the pitch strain; and γ_(β) is the flap strain.
 6. A method as recited in claim 5, wherein the thickness of each layer is varied until the Pitch Strain, the Pitch Strain (γ_(θ)) is approximately 35% for all layers.
 7. A method as recited in claim 5, wherein C is between 4000 and
 6000. 8. A method as recited in claim 5, wherein C is ˜5000.
 9. A method as recited in claim 5, wherein uniform pitch strain is calculated to satisfy: $G_{i} = \frac{G_{i - 1}R_{i - 1}^{3}\varphi_{i - 1}\sin \; B_{i}}{R_{i}^{3}\varphi_{i}\sin \; B_{i - 1}}$ where: G_(i) is the elastomer layer shear modulus; G_(i-1) is the previous elastomer layer shear modulus (starting at the layer closest to the focal point); R_(i) is the mean radius of the layer, R_(i-1) is the mean radius of the previous layer (starting at the layer closest to the focal point), and φ_(i)=cos A _(i)·(sin A _(i))²+2·cos A _(i)−cos B _(i)·(sin B _(i))²−2·cos B _(i) where: A_(i) is the Inner Angle and B_(i) is the outer angle. to produce a uniform fatigue life of each bearing layer.
 10. A method as recited in claim 7, wherein the thickness of each layer is varied until the Pitch Strain (γ_(θ)) is approximately 35% for all layers.
 11. A method of calculating a uniform life for a spherical elastomeric bearing comprising the steps of: 1.) Determining Loads and Motions; 2.) Determining Fatigue Life Desired; 3.) Entering Bearing Dimensions; 4.) Entering Bearing Elastomer Material Properties; 5.) Calculating Geometries; 6.) Calculating Shape Factors Per Layer; 7.) Calculating Effective Compression Modulus (Ec) Per Layer; 8.) Calculating Radial And Axial Strains To Get Compression Shear Strain Per Layer; 9.) Calculating Stiffnesses (Radial, Axial, Flapping, And Torsional Stiffnesses); 10.) Calculating Torsional Strain and add layers to achieve allowable torsional strain; 11.) Calculating the Shear Modulus to Achieve A Uniform Torsional Strain; 12.) Calculating a Flapping Strain; 13.) Calculating Shear Strain Allowable; 14.) Calculating Life; and 15.) Varying the Thicknesses Of Each Layer To Meet Shear Strain Allowable to Achieve Uniform Life.
 12. A method as recited in claim 11, wherein said Determining Loads and Motions further comprise: a. Determine the axial load; b. Determine the radial load; c. Determine the vibratory pitch motion (theta) angle; d. Determine the flapping (cocking) motion angle.
 13. A method as recited in claim 11, wherein said Entering Bearing Dimensions further comprise: a. Inner radius (Ri), Inner Angle (Bi), Inner Angle (Ai), Side Angle (Alpha); and Side Angle (Blpha); b. Enter Shim thickness.
 14. A method as recited in claim 11, wherein said Entering Bearing Elastomer Material Properties further comprise: a. Entering Bulk modulus of elastomer, (Kb, psi); and b. Entering the first layer shear modulus (G, psi).
 15. A method as recited in claim 11, wherein said calculating radial and axial strains to obtain compression shear strain further comprises: a. changing a layer thickness to achieve allowable compression strain. 